It is necessary to monitor gases from smoke stacks in industrial plants to be sure that the levels of TRS and SO.sub.2 are within governmental performance specifications. One device which has been used for this purpose is the "TRS Extractive System" manufactured by Lear Siegler Management Controls Corporation of Englewood, Colorado. In this device, the gas sample extracted from the stack is split into two streams. One stream flows continuously through a reference section of a sample cell, providing a reference, or common mode SO.sub.2 level equal to the process SO.sub.2 concentration. The other portion of the extracted gas stream passes through a thermal oxidizing furnace where TRS compounds are oxidized to SO.sub.2. The gas stream exiting the furnace contains the process SO.sub.2 concentration plus the SO.sub.2 resulting from the oxidized TRS. This stream flows continuously through the measurement section of the cell. The detector then senses the difference in attenuated light intensity as the light beam is alternated between the measurement and reference portions of the sample cell. The difference signal is proportional to the SO.sub.2 produced by oxidizing the TRS compound. Thus, the analyzer measures the TRS concentration directly in the presence of a high common mode level of SO.sub.2 without measuring the individual SO.sub.2 levels. The instrument can be automatically adjusted to compensate for the effects of line voltage variation, lamp and detector aging, and varying concentrations of background SO.sub.2. However, changes cannot be measured in the precipitate that collects in the cells. Since each cell will have contaminate build-up at different rates, over a period of time the accuracy of the device will decrease.
Another type of analyzer is one in which the sample gas is diluted and then passed through an SO.sub.2 scrubber and oven before being introduced into a single SO.sub.2 analyzer. While this apparatus overcomes the disadvantage of using two cells, the SO.sub.2 scrubber is quite expensive and must be replaced frequently because of the levels of SO.sub.2 found in TRS monitoring applications.